The nomenclature for silicone elastomers and resins is complicated and often confusing. Two key raw materials are dimethicone/ vinyl dimethicone copolymer and bis-vinyldimethicone/dimethicone copolymer. These products are defined by the reactants used to make them rather than their specific structure.

Dimethicone/vinyl dimethicone copolymer (CTFA Monograph ID 9792) is described as a copolymer of dimethyl polysiloxane crosslinked with vinyl dimethylpolysiloxane. Bis-vinyldimethicone/dimethicone copolymer (CTFA Monograph ID 20642) is described as a copolymer of dimethicone (end blocked) with vinyl dimethicone.

The reaction used to make both these materials is hydrosilylation, shown in Figure 1, whereby a silanic hydrogen compound Si-H is reacted with a vinyl silicone to make a silicone polymer having two methylene groups between two silicon atoms. The product does not contain vinyl groups; rather it contains two reacted methylene groups. It is not made by reacting dimethicone, but by reacting dimethicone with silanic hydrogen.

The name does not indicate whether the vinyl compounds are internal (see Figure 2) or terminal (see Figure 3); the terminal vinyl compound is, however, the material most commonly used in this class of compounds.

Not withstanding these inconsistencies, the salient difference is that the bis-vinyldimethicone/dimethicone copolymer is made with terminal silanic hydrogen dimethicone (Figure 4) rather than internal silanic hydrogen compound (Figure 5).

The result is that there is no branching in the bis-vinyldimethicone/dimethicone copolymer, whereas the dimethicone/vinyl dimethicone copolymer has branching. Neither name indicates if the product is fluid, elastomeric or resinous.